1. Field of the Invention
The present invention relates to a method for making a photosensitive planographic printing plate, and specifically relates to a method for making an infrared laser-sensitive positive photosensitive planographic printing plate for so-called direct plate-making, wherein the printing plate is directly made based on digital signals output from a computer or the like.
2. Description of the Related Art
With the remarkable progress of lasers in recent years, high-power, small-sized solid lasers and semiconductor lasers having emission regions in the near infrared to infrared regions are now readily available. These lasers are extremely useful as light sources for making printing plates directly from digital data output from computers or the like.
Infrared laser-sensitive positive photosensitive planographic printing plates are known as planographic printing plates which are exposed to an infrared laser having an emission region in the above-described infrared region. An infrared laser-sensitive positive photosensitive planographic printing plate is composed of a hydrophilic base having thereon a photosensitive layer composed essentially of a binder resin which is soluble in an aqueous alkali solution, and an infrared absorbing dye (IR dye) which absorbs light to generate heat. Before the infrared laser-sensitive positive photosensitive planographic printing plate is exposed, the infrared absorbing dye interacts with the binder resin to serve as a dissolution inhibitor for the binder resin. Once the photosensitive layer is exposed to infrared rays, heat generates in the infrared exposed areas (non-image areas) which are the exposed area, and the generated heat weakens the interaction between the infrared absorbing dye and the binder resin to solubilize the binder resin in the alkali developer. Accordingly, when the infrared laser-sensitive positive photosensitive planographic printing plate after exposure is developed with an alkali developer, the infrared exposed areas (non-image areas) are dissolved in the alkali developer, whereby an image is formed according to the laser exposure.
However, under the system, the difference in the degree of insolubility of the unexposed areas in the developer and solubility of the exposed areas is so small that excessive development and development failures have tended to occur. In addition, minute surface defects caused during handling may result in defective printing durability. Japanese Patent Application Laid-Open (JP-A) No. 11-218914 proposes a multilayered photosensitive planographic printing plate composed of a hydrophilic base having thereon an undercoat layer which contains an acrylic resin having an alkali-soluble group such as a sulfonamide group, and a top layer which contains a novolac resin and a photothermal converting agent, wherein the top layer increases in solubility in alkaline water upon exposure. In the planographic printing plate of this type, the top layer in the infrared exposed areas is dissolved and removed by alkaline water to expose the undercoat layer having high alkali solubility, which results in notable difference in the solubility of the unexposed and exposed areas.
In general, a photosensitive planographic printing plate is made by subjecting a belt-like aluminum web to various surface treatments, applying a photosensitive layer forming solution to the roughened surface of obtained web, and then drying the coating. The above-described multilayered infrared-sensitive planographic printing plate is obtained by applying an undercoat layer and a top layer one by one to form a photosensitive layer. In order to form the multilayered structure, the acrylic resin composing the undercoat layer must be poorly soluble in the solvent applied to the top layer. Therefore, the solvent composing the undercoat layer is commonly a solvent having a high dissolving power on a poorly soluble resin, and examples thereof include γ-butyrolactone and dimethyl sulfoxide (see JP-A No. 2004-205720).
However, the multilayered infrared laser-sensitive positive planographic printing plate has problems such as the minor differences in sensitivity between the end and center portions in the width direction of the aluminum web, and variations in sensitivity due to the changes in conditions such as the thickness of the aluminum web. These phenomena may result in the variation in the dot size after plate making to cause printing problems.